1. Field of the Invention
This invention relates to high vacuum water vapor cryopumps.
2. Description of the Prior Art
High vacuum chambers used for production processes, as well as research and development, are evacuated by a variety of means including mechanical or sorption "roughing" pumps for preliminary stages, and then diffusion-, turbomolecular-, ion-, titanium sublimation-, and most recently, helium refrigeration (cryo)- pumps. Those in the latter group are considered high vacuum pumps which are suitable for operation in the deeper vacuum range after the chamber has been "roughed" down to a "crossover" pressure. This "crossover" pressure depends upon the gas load tolerance of the high vacuum pump. The high vacuum pump is started long before the "crossover" pressure is reached, and is usually operated continuously, but isolated from the chamber by a large aperture valve located in a suitable manifold or port. When the high vacuum isolation valve is opened and "rough" pumping is stopped, the chamber is pumped at a high rate by the high vacuum pump to its operating pressure level.
Gas loads in the chamber vary considerably from one application to another. Initially, the air gases including nitrogen, oxygen, and water vapor are removed. Later in the pumpdown process, water vapor becomes the dominant gas due to desorption from internal surfaces. Some plasma processes require introduction of inert gases such as argon and reactive gases such as oxygen or halogens. In such cases, it is desirable to pump water vapor at high speeds, without removing the introduced gases too fast, that is, to selectively pump the gases. Water vapor can dissociate and create undesirable oxygen and hydrogen gases.
Pumps vary in their ability to remove different gas species, depending upon their operating principles. A combination of more than one type of high vacuum pump may be desirable. One solution to the problem of high water vapor loads was introduced in the 1950s by C. R. Meissner. A light weight coil of tubing, placed directly in the vacuum chamber, is cooled by liquid nitrogen flowing through it. A Meissner coil cryopumps water vapor at high rates. Unfortunately, it also cryopumps CO.sub.2. This disadvantage is discussed in more detail hereinbelow. The coil must be quickly warmed when the vacuum chamber is to be opened to atmosphere to preclude moisture condensation from the room. This is usually done using heated and pressurized nitrogen gas to expel liquid nitrogen and to warm the coil. After the chamber is unloaded, reloaded, and roughed down, the Meissner coil is quickly recooled concurrent with opening the high vacuum valve and using the high vacuum pump for pumping of the chamber.
Another method of removing water vapor at high rates, with limited pumping of gases such as argon, uses a modified helium cryopump with a throttle attached to the warmer first refrigeration stage. The throttle is cold enough to cryopump water vapor, but permits argon and other "permanent" gases to pass on (at restricted flow) and be pumped by first and second stages.
Most high vacuum pumps are not capable of being started at atmospheric pressure but rather they must be isolated from the chamber by a valve. The valve, a manifold, if present, and aperture between the chamber and pump all reduce gas conductance to the pump with a resulting pumping speed reduction. Larger or additional pumps, can be added to handle large pumping loads. Usually, the largest load (65-95%) is water vapor, therefore gas pumping speeds do not accurately match the load.
Meissner coils, which are placed directly in chambers and used as supplemental pumps for cryopumping water vapor, are costly to operate because of continuous and wasteful nitrogen consumption, have some inherent safety problems, and are difficult to warm up to room temperature in less than five minutes. Also, they operate so cold that they cryopump or cryotrap some CO.sub.2 vapors at typical processing pressures. The chamber pressure then can seriously fluctuate if the coil temperature varies more than 0.1 deg C. It is difficult to control surface temperatures this close, and liquid entrainment in exhausting vapors occurs as a consequence.
Helium cryopumps pump water vapor about three times as fast as air, or 31/2 times as fast as argon, but still not proportional to typical gas loads. Also, because they are capture pumps, they must be periodically regenerated to dispose of captured gases. A total regeneration cycle requires about three to four hours and the chamber can not be used during this period. Care must be taken in removal of gases from the cryopump during regeneration. Cooling capacity is quite limited and they are unable to handle significant thermal radiation heat loads, e. g., viewing surfaces above about 50.degree. C. Periodic changing of a helium purifying cartridge is also required. Helium cryopumps with refrigerated throttle devices have two potential problems: (1) cryopumping action of water vapor is far removed from the vapor source and is somewhat conductance limited by an aperture and high vacuum valve, and (2) the throttle limits the pumping of all gases including hydrogen and oxygen which is undesirable.